In most computer tomography (CT) scanners, the X-ray beam is polychromatic. Third-generation CT scanners generate images based upon data according to the energy integrated nature of the detectors. These conventional detectors are called energy-integrating detectors and acquire energy integrated X-ray data. On the other hand, photon-counting detectors are configured to acquire the spectral nature of the X-ray source, rather than the energy integrated nature. To obtain the spectral nature of the transmitted X-ray data, the photon-counting detectors split the X-ray beam into the X-ray beam's component energies or spectrum bins, and count a number of photons in each of the bins. The use of the spectral nature of the X-ray source in CT is often referred to as spectral CT. Since spectral CT involves the detection of the transmitted X-ray at two or more energy levels, spectral CT generally includes dual-energy CT by definition.
Photon-counting detectors in CT imaging systems are often produced from semiconductor materials such as Cadmium Zinc Telluride (CdZnTe), often referred to as CZT, Cadmium Telluride (CdTe), and Silicon (Si), among others. Semiconductor radiation detectors, e.g., CdZnTe, HgI2, and TlBr, have ideal characteristics to be used as imaging detecting devices: high effective atomic number (Zeff) and high density (ρ). These characteristics provide a high probability for photoelectric absorption of incoming radiation photons, i.e., X-rays, thereby providing a high level of intrinsic detection efficiency. For example, the probability of the photoelectric effect occurring is measured by the cross section of interaction a given by
  σ  ∝            Z      eff      n              E      3      where n is a number that varies between 4 and 5. These characteristics are especially important when the physical size of the imaging detecting devices (detectors) needs to be reduced while maintaining the required detection efficiency.
Using sparse fourth-generation photon-counting detectors combined with third-generation detectors is a viable solution for future hybrid spectroscopic CT systems. This configuration requires the fourth-generation photon-counting detectors to be manufactured in a slender geometry, having one-dimensional arrays of detectors with segmented anodes in the slice direction. In this scenario, it may be required to maximize the intrinsic detection efficiency while reducing the overall dimensions of the fourth-generation photon-counting detectors (PCDs) in order to minimize the effective shadow on the third-generation detectors.
As shown in FIG. 12, the intrinsic detection efficiency can be maximized by increasing the thickness T while reducing the width W in order to minimize the effect of shadows onto the third-generation detectors. This would require semiconductor radiation detectors having a slender geometry having very small widths W (<2 mm). However, the slender shape of the detectors (small radiation detector widths W<2 mm) can cause severe deformation of the operating electric field near the lateral surfaces of the detector. As shown in FIG. 13, deformation of the operating electric field causes charge losses near the surface of the semiconductor, degrading the energy resolution of the detector. Moreover, charge loss at the surface of radiation detectors is worse for small detector widths due to the dipole effect.
Direct-conversion photon-counting detectors (PCDs) are sensitive to light and have to operate in a light-sealed environment. Furthermore, PCD systems that include ASICs to control the PCDs are sensitive to electromagnetic interference, and need to be operated inside a Faraday cage. The Faraday cage serves both as an electromagnetic shield and a light shield. However, the Faraday cage, which is biased at certain electrical potentials, can alter the operating electrical potential of the PCDs.